1. Field of the Invention
The subject invention generally pertains to fabric air ducts and more specifically to a damper for such a duct.
2. Description of Related Art
In HVAC systems (heating, ventilating, air conditioning), conditioned supply air discharged from a blower is often conveyed to various rooms or areas within a building by way of ductwork. Conventional sheet metal ductwork may include a main header duct that receives the forced air from the blower and distributes the air onto several branch ducts. The branch ducts, in turn, include one or more discharge registers that deliver the air to the various designated areas.
To ensure that each branch duct receives an appropriate volume of air to adequately condition or ventilate each room or area, airflow control dampers are often installed within the branch ducts, upstream of the ducts' discharge registers. Partially closing a damper prevents its respective branch duct from starving other branch ducts of their supply of air. The various dampers are adjusted until the supply air to each of the branches is properly apportioned, which is a process known as balancing the airflow.
In addition to dampers disposed within the ducts, in some cases, additional dampers are installed at each discharge register. The dampers at the discharge registers allow more individualized control of airflow through each register or allow a register to be shut off completely. The occupants of the building typically adjust the individual dampers at each register, while the other dampers within the ducts are thermostatically controlled or manually adjusted and set when the HVAC system is first installed.
Balancing the airflow is readily accomplished when the ductwork, dampers and registers are all made of relatively rigid sheet metal; however, in many cases, air ducts are made of fabric. Fabric ducts typically have a flexible fabric wall that is porous and/or includes additional holes along its length for evenly dispersing air, from within the duct, to the areas being conditioned or ventilated. An example of such a duct is a DUCTSOX by the Frommelt Safety Products Corporation of Milwaukee, Wis. Fabric ducts are often suspended from a horizontal cable or track by way of several hangers distributed along the length of the duct. Fabric is often preferred over sheet metal when cleanliness, even air dispersion, condensation control, or appearance is a significant concern. Unfortunately, using conventional metal dampers within fabric ducts creates some problems.
First, the pliability of fabric may inhibit the duct from effectively supporting the weight of a metal damper without excessive distortion or sagging of the duct. Second, the supply air blower turning on and off to meet the conditioning demand of the building causes a fabric duct to alternately inflate and deflate. When the duct is deflated, a metal damper may create an unsightly bulge in the duct.
Fabric ducts are also affected by problems during the initial operation of the duct. Unlike metal ducts, fabric ducts maintain their inflated shaped only when they are receiving airflow from the blower. When there is no airflow, the fabric duct is in a collapsed state because there is no static air pressure in the fabric duct. The fabric duct also experiences a shrinkage in that its distal length is shorted somewhat as the duct is in a recoiled position, in comparison to its length when fully inflated. From the shrunken and collapsed position, when the airflow is initiated, the blower feeds a large stream of air that must eventually erect the entire fabric duct. The airflow is typically quite high and as it fills the fabric duct the most distal end of the duct, farthest away from the blower, pops out into the erect position. A large popping sound results. Not only is the popping sound annoying to personnel nearby, the violent fabric duct movement that causes the sound may cause wear over time.